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A year after the return of samples from the asteroid Ryugu to Earth by the Japanese probe Hayabusa-2, the researchers are delivering their first analyzes. In particular, they allow us to understand the composition of the dust from which the objects of the inner solar system were formed. A first step before more in-depth analysis of carbon-rich space rock.
As a reminder of the previous events, on December 6, 2020, the Japanese probe Hayabusa-2 threw 5.4 grams of dust from the asteroid Ryugu in a desert area of South Australia, after traveling millions of kilometers. Ryugu is about 1 kilometer in diameter (with a crest around its equator) and orbits at a distance of 140 to 210 million kilometers from the Sun, in an orbit fairly close to that of Earth.
The mission of reaching the asteroid, landing there twice (February 22 and July 11, 2019), and then bringing the recovered dust back to Earth, required an impressive level of skill and planning. Hayabusa-2 now has an encounter with other asteroids for the next several years.
Asteroids: Good Witnesses of Planet Formation
You should know that asteroids are good witnesses to find the recipe for the formation of planets, and therefore of the Solar System. Ryugu is only the second asteroid from which a sample return mission has been conducted. The first was Itokawa, whose sample return failed, so only a small amount of dust eventually reached Earth in 2010.
In two articles published on December 20, 2021 in Nature Astronomy, international research teams revealed that Ryugu is a very dark and very porous space rock, incorporating some of the most primitive materials in the Solar System that we have ever had. . Although expected, these results are very interesting since the asteroid has remained more or less unchanged since the formation of the solar system 4.5 billion years ago. The exhibition could thus contribute to reviewing the paradigms of the origin and evolution of the solar system.
Ryugu: very dark and porous
On the analysis side, it was already known that Ryugu was a type C asteroid: the most common in the solar system. Its rocks have a high carbon content, which makes them very dark. They also contain many volatile elements. The first Nature article describes microscopic weighings and measurements of grains of different sizes, while the second provides the first information on the composition of these grains.
Ryugu samples brought in by the Japanese probe. © Yada and. al., 2021.
In the first study, led by astronomer Toru Yada, analysis of a sample from Ryugu confirms that the asteroid is extremely dark, undoubtedly one of the darkest in the Solar System. In general, C-type asteroids have an albedo (a measure of the amount of solar radiation reflected by a body) of 0.03 to 0.09. Ryugu’s albedo is 0.02, which means that it reflects only 2% of the solar radiation that hits it.
The second observation concerns the density of this material, which is generally quite low. Therefore, the asteroid is more porous (46%) than the average of the asteroids already studied, which makes it quite fragile. These results correspond to the porosity of the rock measured by remote thermal images and measurements made on the asteroid itself.
In the second article, a team led by astronomer Cédric Pilorget from the University of Paris-Saclay analyzed the composition of the dust. They also detected that the asteroid appears to consist of an extremely dark matrix, probably dominated by phyllosilicates or clay minerals. It would be formed by a hydrated matrix, in which there is organic matter, but also carbonates (a set of carbon and oxygen) and nitrogenous compounds. The microscope used makes it possible to determine which wavelengths are reflected or absorbed, which provides information on the nature of the material present.
Witness of the first ages of the Solar System
These two articles agree that, in terms of porosity and composition, Ryugu is similar to a type of meteorite classified as “CI chondrites.” This means that the space rock is carbonaceous and looks like the Ivuna meteorite (which fell in 1938 in Tanzania). Compared to other meteorites, these have a composition very close to that of the solar photosphere, suggesting that they are the most primitive of all known space rocks.
“Our first laboratory observations for all returned samples demonstrate that Hayabusa-2 recovered a representative, untreated (albeit slightly fragmented) sample from Ryugu,” Yada’s team wrote in their paper. No doubt, more analysis will be carried out at Ryugu to find out more about what our solar system was like when it formed from the dusty remnants of the Sun.
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